A metal core circuit board includes a metal plate with good thermal conductivity embedded in an insulating substrate, thereby improving heat uniformity and heat dissipation. The embedded metal plate improves a thermal characteristic of the circuit board. Hence, it is possible to flow a larger amount of an electric current even in a similar circuit pattern, thereby reducing a size of a circuit, a peripheral component, or the like.
FIG. 15 is a cross sectional view showing an example of the metal core circuit board. In the figure, 11 designates an insulating substrate (a prepreg hardened through pressing and heating); 12 designates a metal plate embedded in the insulating substrate 11; 13 designates a circuit pattern formed on a surface of the insulating substrate 11; 14 designates a through hole plating; and 15 designates a solder resist.
The metal plate as a core of the metal core circuit board is formed of a copper plate, a copper alloy plate, an aluminum plate, an aluminum alloy plate, or the like, preferably a copper plate in view of thermal conductivity. When the core is formed of the copper plate, the core selectively has a thickness approximately between 100 μm and 500 μm, so that the metal core circuit board has sufficient heat uniformity and heat dissipation.
Soldering is performed onto the metal core circuit board with a reflow furnace or the like in a component mounting process. During the process, it is necessary to prevent an interface between the core metal plate 12 and the insulating substrate 11 from being delaminated due to heating. Further, it is necessary to provide the core metal plate and the insulating substrate with sufficient adhesion and heat resistance against heat generated in use.
In general, a copper foil used as a circuit conductive member of a circuit board is an electrolytic copper foil having a roughened surface on one side and a gloss surface on the other side. Normally, the roughened surface is formed through electroplating, and copper plating or the like is performed on the roughened surface to grow a protrusion with a fine bump shape, thereby producing the electrolytic copper foil. FIG. 16 shows a scanning electron micrograph of the roughened surface of the electrolytic copper foil for the circuit board. The copper foil formed through electroplating has a thickness between 35 μm and 70 μm. When the copper foil has a thickness greater than the range, it takes a long period of time for plating, thereby increasing cost for a practical use. Further, the core of the metal core circuit board needs to have the roughened surface on both sides. In order to use the electrolytic copper foil as the core metal plate, it is necessary to perform an additional roughening treatment on the gloss surface thereof, thereby further increasing cost.
In many cases, a large number of holes are formed in the core metal plate of the metal core circuit board for forming through holes connecting circuit patterns on both surfaces. After performing the roughening treatment on the both surfaces of the copper plate without holes, when holes are formed in the copper plate, the roughened surfaces may be damaged. Further, it is necessary to clean and remove machine oil attached to the copper plate when holes are formed. Accordingly, it is desirable to perform the roughening treatment on the both surfaces after forming the holes for the through holes in the copper plate.
In view of the reasons mentioned above, it is difficult to use the electrolytic copper foil for the core metal plate of the metal core circuit board.
When a rolled copper plate capable of being produced with a rolling mill roll at a relatively low price is used for a copper plate with a thickness greater than 100 μm, i.e., a preferred thickness for the core of the metal core circuit board, it is possible to significantly reduce cost. However, the rolled copper plate has smooth surfaces on both sides. Accordingly, when the rolled copper plate is embedded in an insulating substrate, it is necessary to perform the roughening treatment on both surfaces thereof for enhancing adhesion to the insulating substrate (a glass epoxy board).
A process for roughening the surface of the copper plate includes etching, a chemical treatment, or the like. A typical example is CZ treatment of MEC COMPANY LTD. In the CZ treatment, a copper etching particle and a organic nitrogen compound coating film are formed on the surface of the copper plate using a oxidation reduction reaction of a bivalent copper compound, so that it is possible to obtain a peeling strength between 0.4 kN/m and 0.8 kN/m according to JIS-C6471. When the peeling strength in the range is obtained, it is possible to satisfy a minimum requirement for the roughening process for adhering to the insulating substrate.
There has been known a process for forming a protrusion with a fine bump shape on a surface of a copper plate through a copper plating process at a high current density such as a copper foil of a printed circuit board or the like (refer to patent document 1). The protrusion with the fine bump shape has an anchoring effect. Accordingly, it is effective for enhancing adhesion to the insulating substrate as opposed to etching, chemical treatment, or the like.    Patent Document 1: Japanese Patent Publication No. 2005-008973